Umbilical cord blood (UCB) is currently considered an alternative to bone marrow as a source of hematopoietic stem cells for transplantation and it use in adults with hematologic cancers who lack an HLA- matched bone marrow donor. The advantages of UCB are the immediate availability of cells, the absence of risk to the donor and a lower need to HLA compatibility between donor and recipient. Although safety and efficacy of this approach is well documented, no studies have addressed the status and the mechanisms of immune reconstitution in these patients. Early studies with single UCB transplants have demonstrated reduced GvHD in unrelated UBC transplant recipients compared to unrelated BMT recipients. In contrast, infection related morbidity and mortality appears to be higher in UBC recipients compared to unrelated BMT recipients. The mechanisms responsible for this distinct outcome of UBC transplantation have not been determined. We have recently succeeded to isolate regulatory T cells (Treg) and to generate Treg cell lines from UCB. We found that UCB-Treg lines exhibited more potent suppressor activity compared to PBMC- derived Treg lines and induced 95% suppression of allogeneic MLR. Furthermore, our studies revealed that UCB-Treg lines had functional and molecular properties of T cell anergy, including defective activation of Ras-MEK-Erk1/2, enhanced activation of Rap1, and upregulation of p27kip1. Thus, UCB contains a significant number of Treg with a molecular profile of T cell anergy and potent suppressor function. An intriguing hypothesis raised by our findings is that Treg may be responsible for the unique outcome of UCB transplantation. As UCB T cells repopulate the recipient after transplant, it is possible that these patients will have highly potent Treg originated from the UCB graft. After encounter of recipient's alloantigens these Treg will be activated, expand in vivo and inhibit responses of host-specific effector T cells, thereby preventing GvHD. However these UCB derived Treg may also inhibit responses of pathogen-specific and tumor-specific effector T cells and compromise pathogen-specific and potentially tumor-specific immunity. The goal of the present proposal is to determine the role of Treg in the reconstitution of pathogen-specific immunity, GvH-specific and GvL-specific T cell responses after UCB transplantation. A phase II study of sequential transplantation of two unrelated cord blood units is in progress at Massachusetts General Hospital/Dana Farber Cancer Institute. Using material prior and after transplant from these patients we shall undertake the following studies to evaluate the role of Treg on: 1) Recovery of pathogen-specific immunity 2) GvH-specific and GvL-specific T cell responses.